The purpose of this study is to determine whether menopause and/or hormone replacement therapy affects the activity of cytochrome P450 3A4. We measured the pharmacokinetics of prednisolone and erythromycin in post-menopausal women compared to pre-menopausal women controls. In addition, we studied a group of post-menopausal women both before an dafter a two month course of progesterone therapy.